At the moment I am trying to implement a variable that is generally available within OpenFOAM. I have even gone as far as thinking about it being implemented as a pointer only.
Nevertheless all the ideas come back to some crucial point: Is there anything like a "master-file" that contains something like the "main()"-function commonly seen in C++?

A second thought was to orient myself at the implementation of the variable "psi", which seems to be the variable that is solved for in any of the equations. (Am I right at that point?) But again: Where is this variable defined? Then I could use the same file as some kind of blueprint for implementing my variable that would be available at least at most points of the code...